Journal of Surveying Engineering

An improved geoid model for Taiwan was developed using land and sea gravity anomalies and altimeter-derived geoid gradients by least-squares collocation. The estimated model accuracy ranges from 2 cm in the flat area to 10 cm in the mountainous area. This geoid model, along with GPS ellipsoidal heights, is used to determine the errors of the “orthometric heights” at Taiwan’s first-order triangulation stations, yielding an RMS error of 0.97 m. Yushan, east Asia’s highest peak, is now estimated to be 3,950.50 m above the mean level at Keelung from this geoid model and GPS measurements. With recent measured ellipsoidal heights at 33 benchmarks in eastern Taiwan, an average uplift rate of 4.46 cm/year is found along the Longitudinal Valley, and 3.02 cm/year along the coastal highway. This uplift is the result of the collision between the Philippine Sea plate and the Eurasian plate, and the rates are consistent with those derived from terrestrial measurements and tide gauge records.

This paper presents two novel concepts to improve the precision of global positioning system (GPS). First, observations of the low cutoff angle are applied to GPS positioning for obtaining more information to enhance precision. However, unequal weight $P={\cos }^2\left(z\right)$ is used, where z denotes the zenith distance, since the noise of the GPS observations is increased following the increase of the zenith distance. Second, a short-range baseline field is used as the test area. In addition, the external precision of the GPS baseline is evaluated using the distance obtained with a precise telemeter to replace the conventional method, which uses internal precision, i.e., baseline repeatability or standard deviation. The analysis results indicate that the best precision of GPS positioning is obtained at a cutoff angle of 15°. Moreover, using unequal weight to receive low cutoff angle GPS observation data to positioning can improve the precision average 3 mm for our test baselines.